OSError: [Errno 13] Permission denied: '/dev/ttyACM0' - using pyserial from Python to Arduino

Question

Environment

• Linux Mint 17.1
• Python 2.7
• pyserial 2.7
• Arduino UNO rv3

Desired Behaviour

Answer 1

The permissions on the file make no difference to the user that the program runs as

When you are logged in interactively you do have permission to use the /dev/ttyACM0

When your script is running (presumably as the apache user) it does not have permission

You need to alter the permissions on the /dev/ttyACM0

See the 2nd answer here How can I programmatically set permissions on my char device for an example of altering udev permissions so the file has the correct permissions

Answer 2

The following fleshes out some of the ideas in the first answer (I tried to add this content to that answer and accept it, but the edits were rejected). I'm not an expert in the area, so please just use this information to support your own research.

You can do one of the following:

01. Alter the permissions on /dev/ttyACM0 so that world has read and write priviliges (something you may not want to do) - although you may find they reset each time the device is plugged in eg:

sudo chmod 666 /dev/ttyACM0  

02. Create a rule in /etc/udev/rules.d that will set the permissions of the device (a restart will be required):

# navigate to rules.d directory
cd /etc/udev/rules.d
#create a new rule file
sudo touch my-newrule.rules
# open the file
sudo vim my-newrule.rules
# add the following
KERNEL=="ttyACM0", MODE="0666"

This also sets permissions for world to read and write, which you may not want to do.

For more information about this approach, see these answers:

https://unix.stackexchange.com/a/48596/92486

https://stackoverflow.com/a/11848003/1063287

03. The third option, which is the option I implemented, adds the Apache user to the dialout group so that if the script is being run by Apache, then it can access the device.

a) Find the location of your Apache config file, then search for the User setting within that file:

# open file in editor
sudo vim /etc/apache2/apache2.conf
# search for User setting
/User

You may find something like:

# These need to be set in /etc/apache2/envvars
User ${APACHE_RUN_USER}
Group ${APACHE_RUN_GROUP}

b) Quit vim and search for APACHE_RUN_USER in /etc/apache2/envvars (if the above scenario applies):

# open file in editor
sudo vim /etc/apache2/envvars
# search for APACHE_RUN_USER
/APACHE_RUN_USER

You may find something like:

export APACHE_RUN_USER=www-data

c) Add the User www-data to the dialout group:

sudo usermod -a -G dialout www-data

d) Restart.

As the Apache user has been added to the dialout group, the script should now be able to access the device.

Further Reading

How to find the location of the Apache config file:

https://stackoverflow.com/a/12202042/1063287